Device-to-device (D2D) communication between terminal devices in a cellular network relies on the close proximity of terminal devices that are oftentimes close to each other. D2D communication allows terminals devices to operate in a controlled interference environment. Typically, D2D communication shares the same spectrum as the cellular system. For example, some of the cellular uplink resources may be reserved for D2D communication. The other option, i.e., allocating dedicated spectrum resources for D2D communication purposes, is a less attractive alternative because allocating dedicated spectrum resources to D2D communication is undesirable is that spectrum resources are scarce and should be shared when it is practical to do so. Another reason is that dynamic sharing spectrum resources between D2D communication services and cellular services provides more flexibility and improves spectrum efficiency. When spectrum resources are shared between a cellular network and a D2D communication network, the terminal devices in the D2D communication network and the cellular network are aware of each other's uplink subframe timing and can avoid overlapping transmissions on the same uplink resource.
A D2D communication network is also known as an ad hoc network and a D2D terminal device is also known as Proximity Based-Services (ProSe) enabled terminal device. In general, an ad hoc network does not include a central controller. Although only introduced into 3GPP standards recently, ad hoc networks are well-studied and have been deployed in IEEE 802.11 WLAN systems.
In a D2D terminal device, there is the so-called “half-duplex constraint.” That is, a terminal device cannot send and receive D2D communication signals simultaneously. As a result, if two terminal devices in close proximity transmit simultaneously, there is a collision between the transmissions from the two terminal devices. “Collision” in the present application refers to the scenario in which two or more transmitting devices in close proximity are transmitting simultaneously and, as a result, no transmitted signal is received by any of the transmitting devices due to the “half-duplex constraint.” When a collision between two transmitting terminal devices happens, neither terminal device receives the data transmitted by the other terminal device.
The present application discloses advanced methods and apparatus that can be used to resolve collisions between transmissions from D2D terminal devices without relying on a central controller.